This invention relates to an article including an airfoil having an airfoil surface with a thermal insulating outer layer, and more particularly, to such an airfoil with such an outer layer on less than all of the airfoil surface.
Gas turbine engine components operating in the hotter sections of a gas turbine engine are subjected to or experience relatively high temperature oxidizing and corrosive conditions, as well as significant temperature variations across regions of the component. One type of such component is a blading member, including turbine blades, vanes and nozzles which include an airfoil disposed in the hot gas path of the engine. It is common practice in the gas turbine engine art to apply to an outer surface of certain of such airfoils a thermal insulating outer layer, generally in the form of an outer ceramic thermal barrier coating (TBC).
Generally the TBC, when used on a gas turbine engine component, is part of a TBC system. Such a system includes a metallic bond coat disposed on an article substrate, for example an airfoil substrate, for environmental protection and over which is applied the outer ceramic TBC for thermal insulation. A commonly used type of TBC outer layer is a thermal insulating coating based on zirconia stabilized with yttria, for example about 93 wt. % zirconia stabilized with about 7 wt. % yttria. One preferred method for application or deposition of such a TBC coating is by electron beam physical vapor deposition in a chamber in which an article to be coated is disposed and/or moved so that the TBC vapor contacts the surface to be coated. Generally a vacuum or controlled low pressure atmosphere is provided in the chamber. Apparatus of that type is sold commercially for such uses. Other coating application processes, such as plasma spray methods, can be and are used for TBC application in the gas turbine engine art. This general type of TBC has been reported in such U.S. Pat. No. 4,055,705--Stecura et al. (patented Oct. 25, 1977); U.S. Pat. No. 4,328,285--Siemers et al. (patented May 4, 1982); and U.S. Pat. No. 5,236,745--Gupta et al. (patented Aug. 17, 1993).
Evaluation of TBC coated airfoil surfaces of such gas turbine engine articles as turbine blades, vanes and nozzles after service operation in an engine has shown that there is a variation in thermal exposure across the airfoil surface. This results in different combinations of heat loads and stresses, herein called environmental operating conditions, across the airfoil with certain airfoil regions experiencing more strenuous environmental operating conditions than other portions of the airfoil. For example for a typical air cooled turbine blade airfoil, the concave or pressure side of the airfoil and the leading edge between the concave side and the convex or suction side of the airfoil experiences more strenuous environmental operating conditions than does the convex side of the airfoil. This results in an unbalance of such conditions across the airfoil.